Superionic Germanium Substituted Lithium Thioarsante Li_{6+ x}Ge_xAs_{1- x}S₅Br Argyrodites With High Air Stability

Lithium argyrodite solid electrolytes are promising for all-solid-state batteries due to their high ionic conductivity and low elastic modulus. However, poor chemical stability in humid conditions remains a major hurdle towards large-scale practical implementation. This motivates substituting phosphorus with softer acids to stabilize sulfur, where the challenge is to maintain high Li-ion mobility. In this work, we explore the effects of aliovalent (Ge) substitutions on the structure, ionic transport, and air stability of Li₆AsS₅Br. The induced structural modification releases the rate-limiting step in long-range Li-ion transport, as demonstrated by Molecular Dynamics and percolation simulations. As a result, the ionic conductivity reaches 13 mS/cm for Li_6.5As_0.5Ge_0.5S₅Br, a 6-fold improvement over pristine Li₆AsS₅Br. The key enabler for combining high Li-ion mobility and improved stability towards moisture is the increased polarity of the Ge─S bonds compared to As─S or P─S bonds. This reduces Li-ion trapping effectively flattening the energy landscape for diffusion and thermodynamically suppresses H₂S formation upon exposure to moisture, as experimentally demonstrated. Finally, stable cycling performances in combination with high nickel ternary cathodes are demonstrated. Hereby, this research provides a deeper understanding of the role of composition on the ionic conductivity and moisture stability of lithium argyrodites.